Metal Machining - Theory and Applications Episode 1 Part 8 ppsx

a ductile cast iron, shows ceramic tools having a larger n-value than a carbide tool – for the white alumina, n ≈ 1.2. This very high value indicates a mechanical wear mechanism reducing in intensity with increasing cutting speed. Figure 4.14(b), for turning two differ- ent Ti-alloys with a carbide tool, is an example of where a break point falls within the prac- tical cutting speed range. Figure 4.14(c), for face milling a grey cast iron, shows a condition in which tool life decreases with reducing cutting speed. Taylor’s equation – influence of feed and depth of cut Tool life is influenced by feed and depth of cut, as well as by cutting speed. Additional life equations are fT n 2 = C 2 ; dT n 3 = C 3 (4.3) and these may be combined with equation (4.2) (replacing n there by n 1 ) to give V 1/n 1 f 1/n 2 d 1/n 3 T = C′ (4.4) When tool life is limited by thermal damage mechanisms, n 1 < n 2 < n 3 : i.e. cutting speed has a larger influence on life than does feed than does depth of cut, reflecting the influences of these variables on cutting temperature. If, however, tool life is determined by chipping and fracture failures, n 2 and n 3 can become smaller. 4.2.3 Tool life fluctuations It is almost impossible to keep cutting conditions exactly constant in practical machining. Even if it were possible, it would be found that tool life and failure are phenomena based on probability. Fluctuations cannot be avoided in these. However, the range of fluctuations is influenced by the damage mechanism. It is easy to imagine larger fluctuations when chipping, or fracture rather than abrasion is the main mechanism. Figure 4.15 shows the cumulative probability of flank wear development after 1 min of Tool life 133 Fig. 4.15 Distributions of flank wear after turning free-cutting steel B1112 and difficult-to-cut sintered steel and Inconel 718 0.05 0.02 0.1 0.2 0.5 1.0 2.0 5 10 20 40 60 80 99 85 Flank wear VB (mm) P10 Sintered steel P10 B1112 TiC-Al 2 O 3 ceramic Inconel 718 VB max B112 - P10, V = 200m/min, d = 0.5mm, f = 0.1mm/rev Sintered steel - P10, V = 200m/min, d = 0.5mm, f = 0.1mm/rev Inconel 718 - Al 2 O 3 -TiC ceramic, V = 200m/min, d = 0.5mm, f = 0.19mm/rev Cumulative probability (%) Childs Part 1 28:3:2000 2:41 pm Page 133 turning a resulphurized free machining steel and a sintered steel with a P10 carbide tool (plotted on a Weibull chart). Abrasion was the main cause of tool wear with the free machining steel, while edge chipping was the mechanism with the sintered steel. The different slopes of the Weibull plots are clear. The figure also shows the distribution for turning Inconel 718 with an Al 2 O 3 /TiC ceramic tool. As well as the greater amount of wear, the similarity of slope between this and the sintered steel observations is striking. Figure 4.16 is an example of tool wear and wear distribution influenced by the machine tool. It gives the results of face milling a quenched die steel with an Al 2 O 3 /TiC ceramic tool, in the same conditions apart from the machine tool used. Tool wear was by edge chip- ping or fracture. Machine B obviously provides better resistance against this type of damage. This is due to a better stiffness, maybe a better dynamic stiffness. 4.3 Summary This chapter complements Chapter 3 on tool bulk properties, by focusing on the mecha- nisms of cutting edge damage and their characteristic developments with time. Cutting edges experience much higher normal and shear stresses than almost any other type of bearing surface and, at high cutting speeds, high temperatures are also generated. It is not surprising that tool lives are measured in minutes rather than in hours, and certainly not in days. Abrasion occurs with all tools if the work material has hard enough phases, and self- abrasion follows from other mechanical causes of damage. Mechanical damages, of 134 Tool damage 1000 2000 5000 10 4 2x10 4 5x10 4 99 90 70 50 30 10 3 1 Machine tool A Machine tool B Numbers of impact until tool fracture Cutting speed : 220 m min -1, Depth of cut : 0.1 mm, Feed rate: 0.1 mm/tooth, Cutter Dia. : 80 mm Fracture probability (%) Fig. 4.16 Distributions of tool life limited by fracture when milling a quenched die steel (HRC60) with an Al 2 O 3 /TiC ceramic tool, on two different milling machines Childs Part 1 28:3:2000 2:41 pm Page 134 increasing size – from attrition, to chipping, to fracture – increase the more brittle is the tool material and they are relatively insensitive to temperature changes. Thermal damages follow diffusion and chemical reactions. They are very sensitive to temperature and are particularly variable from one tool and work combination to another. Adhesive wear depends on both mechanical and thermal factors, and passes through a maximum rate as temperature increases. For all these reasons of complexity and further influences of mode of cutting, and of the machine tools themselves, on tool life, it has not been attempted to provide comprehensive guidance on tool damage rates. Rather, the goal has been to emphasize what phenomena can occur, and what their effects look like, so mechanisms limiting life in different circum- stances may be recognized and sensible directions for improved performance may then be investigated. References Cook, N. H. (1973) Tool wear and tool life. Trans. ASME J. Eng. Ind. 95B, 931–938. Dawihl, W. (1941) Die Vorgange beim Verschleiss von Hartmetallegierungen. Stahl und Essen 61, 210–213. Gregory, B. (1965) Surface interaction of cemented carbide tool material and Armco iron. Brit. J. Appl. Phys. 16, 689–695. Kitagawa, T., Maekawa, K., Shirakashi, T. and Usui, E. (1988) Analytical prediction of flank wear of carbide tools in turning plain carbon steels (Part 1). Bull. Jap. Soc. Prec. Eng. 22(4), 263–269. Naerheim, Y. and Trent, E. M. (1977) Diffusion wear of cemented carbide tools when cutting steel at high speeds. Metals Technology 4, 548–556. Narutaki, N. and Yamane, Y. (1976) Wear mechanism of carbide tool based on the reaction between tool and work material (Part 1 – reaction test). Bull. Jap. Soc. Prec. Eng. 10(3), 95–100. Narutaki, N. and Yamane, Y. (1993) High-speed machining of Inconel 718 with ceramic tools. Annals CIRP 42(1), 103–106. Takeyama, H. and Murata, R. (1963) Basic investigation of tool wear. Trans ASME J. Eng. Ind. 85, 33–38. Trent, E. M. (1952) Some factors affecting wear on cemented carbide tools. Proc. I. Mech. E. Lond. 166, 64–74. Trigger, K. J. and Chao, B. T. (1956) The mechanism of crater wear of cemented carbide tools. Trans ASME 78,1119–1126. Uehara, K. (1976) On the generating mechanism of wear particles on carbide cutting tools. J. Japan Soc. Prec. Eng. 42(6), 445–452. Usui, E., Shirakashi, T. and Kitagawa, T. (1978) Analytical prediction of three dimensional cutting process Pt. 3. Trans ASME J. Eng. Ind. 100, 236–243. Yamane, Y. and Narutaki, N. (1983) The effect of atmosphere on tool failure in face milling (1st report). J. Jap. Soc. Prec. Eng. 49(8), 521–527. References 135 Childs Part 1 28:3:2000 2:41 pm Page 135 5 Experimental methods Previous chapters have presented optical and electron microscope pictures of chip sections and worn tools, and the results of cutting force and temperature measurements. In addition to cutting force measurements, acoustic emission is also used to study the health of a cutting process. This chapter explains a number of these experimental methods. 5.1 Microscopic examination methods 5.1.1 The quick-stop technique Direct observations as well as theoretical analyses are needed to clarify chip formation mechanisms. Ideally, such observations would be during cutting, to follow dynamic Fig. 5.1 The principle of a quick-stop device for use in turning Childs Part 2 28:3:2000 3:09 pm Page 136 variations of chip flow. Although video cameras have been used to gain an external overview of dynamic chip motions, and it is possible to look through transparent tools (for example made from diamond or sapphire) directly at the chip contact, it is difficult, in general, to resolve much because of the small scale of the deforming region and usually the high cutting velocities. Experimentalists are prepared to lose dynamic information to gain microscopic detail, by freezing the motion, for later study. The quick-stop technique is a popular method for achieving this. The machining process is stopped quickly, by moving the tool and work material apart at a speed greater – preferably much greater – than the cutting speed. The chip is left attached to the work (sometimes with a fragment of the cutting edge attached as well). The photographs in Figure 2.4 are polished and etched sections of quick-stopped chips. Figure 5.1 is a schematic view of a quick-stop device for use with a stationary tool and a moving workpiece, such as in turning, while Figure 5.2 shows a device that could be used for a stationary work and moving tool, as in milling. In Figure 5.1, the tool is supported at a pivot point and by a shear pin. A mass M is made to strike the tool holder with a speed V M . If the impact force is much greater than needed to break the shear pin, the mass will then cause the tool holder to swing quickly away from the chip. The tool holder’s velocity V T does not instantaneously reach the cutting velocity V that is necessary for cutting to stop, because of its inertia. However, to minimize the retraction time, M and V M should be made large and the inertia of the holder should be made small. In practice, V M is frequently made large by firing the mass M from a gun (although for low cutting speed turning tests, hitting the tool holder with a hammer can be sufficient). A device that uses a humane killer gun (normally used for stunning animals prior to slaughter) with its captive bolt as the mass M was reported to achieve a tool displacement of 2.5 mm in 1.2 × 10 –4 s (Williams et al., 1970). If this is assumed to have occurred at approximately constant acceleration, and it is supposed that, for a successful quick-stop, V T must reach V in a cut distance less than f/10, then this device can be used successfully, provided Microscopic examination methods 137 Fig. 5.2 The principle of a quick-stop device for use in milling Childs Part 2 28:3:2000 3:09 pm Page 137 V[m/min] ≤ 354 ͱ⒓⒓⒓⒓⒓⒓ f[mm] (5.1) For a feed of 0.13 mm, the largest allowable cutting speed is 128 m/min, while for f = 0.5 mm, the largest speed is 250 m/min. These speeds are larger than those represented in Figure 2.4, but are not large compared with what can be of interest in modern high speed machining. The acceleration required of the tool increases as the square of the cutting veloc- ity, so successful quick-stops become rapidly more difficult as the cutting speed increases. A similar discussion could be developed in terms of the device of Figure 5.2. However, in milling, it is more difficult to guide the work material away from the cutting edges, and the work and its holder have higher inertia than the tool and its holder in turning. The quick-stop must be synchronized with the intermittent cutting action. There must be a very special reason to pursue a quick-stop in milling, to make the difficulties worthwhile. Quick-stops can show different results, depending on the adhesion between the chip and the tool (Figure 5.3). If there is low adhesion, a clean separation between the two will occur, as shown in Figure 5.3(a). Coated tools usually show this behaviour. If there is high adhesion relative to the strength of the chip or tool, any of the results of Figure 5.3(b) to (d) can occur. If it is particularly desired to preserve the chip/tool interface, a result like Figure 5.3(d) can be engineered by artificially weakening the tool with a notch or crack on its rake face. 138 Experimental methods Fig. 5.3 Modes of quick-stop separation Childs Part 2 28:3:2000 3:09 pm Page 138 5.1.2 Other chip form and wear observations Careful observation of tools and chips after machining can often reveal useful information, without the need for quick-stops. For example, the built-up edge (BUE) formed in machin- ing is usually unstable. It is carried away on the back surface of chips, so observation of the chips (Figure 5.4) can give information as to whether BUE is formed or not. It is obvi- ous that information about wear is obtained by looking at the cutting tools at any time after cutting. Chapter 4 has shown examples of SEM and EPMA used to study wear and contact conditions in great detail. The magnifications of these techniques are not always necessary. In many cases, a low magnification optical microscope, × 10 or × 20, is enough. Such a microscope on an X–Y measurement stage is commonly used in laboratories or machine shops to record wear images and their sizes. Wet photography and printing paper used to be used for archiving information for many years. Now, a high quality CCD camera and a personal computer with a large memory can do the job. 5.2 Forces in machining 5.2.1 Resultant forces Forces in machining can be measured in two main ways: directly or indirectly. Direct measurements involve mounting a tool (in turning) or the tool or workpiece (in milling) on a dynamometer, which responds to the forces by creating electrical signals in proportion to them. These measurements are used when the forces need to be known accurately both in magnitude and direction, for example if thrust, feed and the main cutting forces in turn- ing are required (Figure 5.5), or the torque and thrust force in drilling are needed. Indirect measurements involve deductions from the machine tool behaviour. For exam- ple, the power used by the main spindle motor increases with the main cutting force or torque; and that used by the feed motions can be related to the feed force. Particularly with Forces in machining 139 (a) (b) Fig. 5.4 The back surface of chips formed from 0.15% C steel by P20 carbide tools: (a) with built-up edge, v = 40 m min –1 , d = 2.0 mm, f = 0.08 mm rev –1 ; (b) without built-up edge, v = 100 m min –1 , d = 2.0 mm, f = 0.12 mm rev –1 . Childs Part 2 28:3:2000 3:09 pm Page 139 NC machines, which are fitted with high sensitivity and response main and feed drive motors, indirect methods can be used to determine the active forces. Indirect methods are less accurate than direct methods, but can be sufficient for monitoring purposes. The main consideration here will be direct methods. Tool dynamometers – general points A tool dynamometer should have high sensitivity, high rigidity, high frequency response, high linearity and low drift. Sensitivity is expressed as electrical output per unit force input. Useful dynamometers must be able to discriminate at least 1% of full scale output. Rigidity depends strongly on the dynamometer’s construction. The force sensing trans- ducer is usually the least rigid element of a dynamometer’s structure: different types of element are considered in the following subsections. Frequency response depends on a dynamometer’s natural frequency and damping char- acteristics. In line with elementary dynamics, these may be described in terms of the response of a viscously damped elastic system subjected to a harmonic forcing system: mx> + cx˘ + kx = P m sin wt (5.2) Figure 5.6 shows how the amplitude ratio (the response relative to the response in static conditions) of such a system varies with frequency ratio (the frequency relative to the system’s undamped natural frequency of ͱ⒓⒓⒓ k/m) and damping factor c/c c , where c c is the critical damping coefficient. The figure shows that for a linear response between amplitude and force (and hence a linear response between a dynamometer’s output and force), a damping factor slightly less than 1, around 0.7, is desirable and then a dynamometer could be used at frequency ratios up to 0.2 to 0.3. 140 Experimental methods Fig. 5.5 Forces acting on cutting tool in turning Childs Part 2 28:3:2000 3:10 pm Page 140 Linearity and drift are usually more influenced by the electrical elements (including signal amplification) than by the mechanical elements of a dynamometer. Systems with linearity better than 0.5% of full scale output are required. Drift, which describes the stabil- ity of output (both from the dynamometer transducer and amplification system) over time, can be a problem with cutting force dynamometers because of the sensitivity of electrical elements to temperature changes and the tendency of machining to heat its surroundings. Strain gauge dynamometers A common type of dynamometer uses strain gauges to sense elastic strains caused by cutting forces. Figure 5.7 shows a basic elastic beam type dynamometer with gauges bonded to its surface. It also shows an example of a wire-type gauge and a Wheatstone bridge and ampli- fier system usually used to measure strain changes in the gauges. The main cutting force F C will cause the beam to bend, so that the gauge on the top surface will be placed in tension, that on the bottom surface will be placed in compression, and those gauges on the side surfaces (at the neutral axis) will experience no strain. Likewise, a feed force will strain the side-face gauges but not those at the top or bottom. The arrangement shown in Figure 5.7 is not sensi- tive to force along the axis of the beam as this causes equal strain changes in all gauges. The fractional resistance change of a strain gauge (DR/R) is related to its fractional length change or direct strain (DL/L) by its gauge factor K s : K s =(DR/R)/(DL/L) (5.3) For wire strain gauges, K s is typically from 1.75 to 3.5. Strains down to 10 –6 may be detected with a bridge circuit. The upper limit of strain is around 2 × 10 –3 , determined by the elastic limit of the beam. A disadvantage of the simple cantilever dynamometer is that the gauges’ strains depend Forces in machining 141 Fig. 5.6 The frequency response of a damped forced vibration system Childs Part 2 28:3:2000 3:10 pm Page 141 basically on the moment applied to the section at which they are positioned. They there- fore depend on the gauges’ distance from where the load is applied, as well as on the size of the load. Better designs, less sensitive to where the load is applied, are the octagonal ring and parallel beam designs shown in Figure 5.8. Supporting the load on well-separated thin sections results in the sum of the strains in the gauges being unchanged when the point of application of the load is changed, even though the strains are redistributed between the sections. It is possible to connect the strain gauges in a bridge circuit so that the output is not sensitive to where the force is applied. The choice of parallel beams or octagonal rings is a matter of manufacturing choice. For both, it is important, as a matter of convenience, to minimize cross-sensitivity between the different orthogonal components of electrical output and mechanical input. For the parallel beam design, this is achieved by manufacturing the two sets of beams perpendic- ular to each other. For the octagonal ring design, it is important to choose a particular shape of octagon. When a circular ring (Figure 5.9) is loaded radially there is zero strain at the positions B and B′, ± 39.9˚ from the point of application of the radial load; likewise when the ring is loaded tangentially, there is zero strain at A and A′, ± 90˚ from the load. Gauges placed at A and A′ will respond only to radial loads; and at B and B′ only to tangential loads. The strains will depend on the loads and the ring dimensions (radius R, thickness t and width b) and Young’s modulus E as 142 Experimental methods Fig. 5.7 A strain gauged cantilever dynamometer with its bridge circuit Childs Part 2 28:3:2000 3:10 pm Page 142 [...]... calibration set-up Childs Part 2 28: 3:2000 3 :11 pm Page 15 1 Temperatures in machining 15 1 Fig 5 .17 A detail of the hot junction and the associated measurement circuit Fig 5 . 18 Calibration test results for P10 carbide and a 0.45% plain carbon steel has been inserted in it It has the advantage that a precise measurement of temperature at the bottom of the hole can be made, relying on the standard thermocouple,... variant (below) Childs Part 2 28: 3:2000 3 :10 pm Page 14 8 14 8 Experimental methods temperature is to be deduced from measurement of the EMF generated Standard material combinations are copper-constantan (60%Cu–40%Ni), chromel (10 %Cr–90%Ni)–alumel (2%Al–90%Ni-Si-Mn) and platinum–rhodium In metal machining applications, it is possible to embed such a standard thermocouple combination in a tool but it is... junctions A, B and C, between the work and slip ring, the slip ring and recorder wire and the tool and the recorder wire, are all at the same (cold junction) temperature, the circuit from A to C is all intermediate and has no effect on the EMF But this is often not the case Fig 5 .14 A tool–work thermocouple circuit Childs Part 2 28: 3:2000 3 :10 pm Page 14 9 Temperatures in machining 14 9 Fig 5 .15 A circuit... split-tool method (Figure 2. 21) , although even this is limited – by tool failure – to studying not-too-hard work materials cut by not-too-brittle tools Figure 5 .12 shows a practical arrangement of a strain-gauged split-tool dynamometer The part B of the tool (tool 1 in Figure 2. 21) has its contact length varied by grinding away its rake face It is necessary to measure the forces on both parts B and. .. A, to check that the Childs Part 2 28: 3:2000 3 :10 pm Page 14 6 14 6 Experimental methods Fig 5 .12 A split-tool dynamometer arrangement sum of the forces is no different from machining with an unsplit tool It is found that if extrusion into the gap between the two tool elements (g, in Figure 2. 21) is to be prevented, with the surfaces of tools A and B (1 and 2 in Figure 2. 21) at the same level, the gap... changes they cause in tools – see Trent, 19 91 – but this will not be covered here.) 5.3 .1 Thermocouple methods Figure 5 .13 shows an elementary thermocouple circuit Two materials A and B are connected at two junctions at different temperatures T1 and T2 The electro-motive force (EMF) generated in the circuit depends on A and B and the difference in the temperatures T1 and T2 A third material, C, inserted...Childs Part 2 28: 3:2000 3 :10 pm Page 14 3 Forces in machining 14 3 Fig 5 .8 Octagonal ring and parallel beam dynamometer designs: (a) Octagonal ring type tool dynanometer; (b) parallel beam type tool dynanometer 1. 09FP eA,A′ = ± ——— Ebt2 2 . 18 Fc eB,B′ = ± ——— Ebt2 } (5.4) The manufacture of the ring outer surface as an octagon... frequency Simple beam Childs Part 2 28: 3:2000 3 :10 pm Page 14 4 14 4 Experimental methods Fig 5.9 The loading of a ring by radial and tangential forces dynamometers, suitable for measuring forces in turning from 10 N to 10 kN, can be designed with natural frequencies of a few kHz The ring and the strut types of dynamometer tend to have lower values, of several hundred Hz (Shaw, 19 84 , Chapter 7) These frequencies... results, for a P10 carbide tool and a 0.45% plain carbon steel work, are given in Figure 5 . 18 Even at 10 00˚C the EMF is only 10 mV, so a high sensitivity recorder is needed Inserted thermocouple measurements Figure 5 .19 shows two further possibilities of tool temperature measurement In Figure 5 .19 (a), a small diameter hole has been bored in the tool and a fine standard thermocouple Fig 5 .16 A tool–work... failure, and friction stresses strongly influence chip formation The possibility of using photoelastic studies as well as split-tool methods to determine tool stresses has already been introduced in Chapter 2 (Section 2.4) The main method for measuring the chip/tool contact stresses Childs Part 2 28: 3:2000 3 :10 pm Page 14 5 Forces in machining 14 5 Fig 5 .10 The principle of piezoelectric dynamometry Fig 5 .11 . sintered steel and Inconel 7 18 0.05 0.02 0 .1 0.2 0.5 1. 0 2.0 5 10 20 40 60 80 99 85 Flank wear VB (mm) P10 Sintered steel P10 B 111 2 TiC-Al 2 O 3 ceramic Inconel 7 18 VB max B 112 - P10, V = 200m/min,. 0.1mm/rev Sintered steel - P10, V = 200m/min, d = 0.5mm, f = 0.1mm/rev Inconel 7 18 - Al 2 O 3 -TiC ceramic, V = 200m/min, d = 0.5mm, f = 0 .19 mm/rev Cumulative probability (%) Childs Part 1 28: 3:2000. mechanism. Figure 4 .15 shows the cumulative probability of flank wear development after 1 min of Tool life 13 3 Fig. 4 .15 Distributions of flank wear after turning free-cutting steel B 111 2 and difficult-to-cut